April 21, 1995

A weekly feature provided by scientists at the Hawaiian Volcano
Observatory.

Fogo Volcano, Cape Verde Islands

A team of volcanologists from the U.S. Geological Survey has been monitoring
an eruption that began just after midnight on April 3 on Fogo in the Cape
Verde Islands off the west coast of Africa. Frank Trusdell from the Hawaiian
Volcano Observatory and Richard Moore (former geologist on the staff of the
Hawaiian Volcano Observatory) from our Denver, Colorado office arrived in Fogo
on April 10 in response to a request from the State Department. Our Volcano
Disaster Assistance Program, funded by U.S. AID and the U.S. Geological Survey,
provides assistance in volcanic crises or emergencies around the world. This
report is based mainly on their daily communications from Fogo, and an earlier
report on the eruption sent to the Smithsonian Institution's Global Volcanism
Network by geologists J. Gaspar and N. Wallenstein from the University of the
Azores. They were on Fogo when the eruption began.

Like Hawaii, Fogo is a "hot-spot" volcanic island. It last erupted in 1951. It
is the youngest and most active volcano in the Cape Verde Islands. The Cape
Verde Islands, a short chain of volcanic islands that generally are younger at
the western end, formed as the African Plate moved towards the east over the
hotspot in much the same way that the Hawaiian Ridge formed as the Pacific
Plate moved west-northwestward over a hotspot.

Unlike the island of Hawaii, Fogo consists of a single volcano, so the island
is nearly round and about 25 kilometers (15 miles) in diameter. The large
summit caldera (about 10 kilometers in the north-south direction and 7
kilometers in the east-west direction) is not located in the center of the
island, but rather towards its northeastern corner. The caldera is bounded by
steep near-vertical fault scarps on the north, west, and south sides but is
breached to the east where lava can flow to the coast. The current
north-northeast-trending eruptive fissures are located along the western flank
of a large cone named Pico, which formed inside the caldera between about 1500
and 1760.

Residents reported that the eruption was heralded by small earthquakes which
began about six days prior to the outbreak of lava. These earthquakes increased
in size and frequency through April 2, and a particularly strong one was
reported at about 8 p.m., some four hours before the eruption apparently began.
Just after midnight, the fissures opened on the flank of Pico. One resident
said it looked as if the cone had "been cut by a knife." The eruption began
with Strombolian (moderately explosive) activity, quickly followed by a
curtain-of-fire fountain that fed a flow which cut off the road to the village
of Portela by 2 a.m. The 1,300 people living inside the caldera fled during the
night to the safety of villages on the north coast. No one was killed; about 20
people required medical attention, although our reports do not indicate the
nature or severity of their injuries. We suspect that they inhaled volcanic
fumes and ash particles, which caused difficulties in breathing.

During the day on April 3, the island was covered by a thick cloud of dark ash
that reached 2.5 to 5 kilometers high. The initial flows were pahoehoe lava,
although their extent was apparently small. The violent eruption produced lava
bombs up to 4 meters across that were ejected as far as 500 meters from the
vents. On April 4, lava fountains reached 400 meters high beneath a cloud of
ashes about 2 kilometers high. A new scoria cone formed that was open to the
southwest and fed a growing 'a'a flow. On April 5, the new 'a'a flow reached
the western caldera wall and destoyed about five houses and the main water
reservoir. During these first few days, civil authorities evacuated about 3,000
people.

The eruption has become more steady following the first few days, and
moderate, Hawaiian-style lava fountains about 100-120 meters high have built a
spatter cone that now stands more than 140 meters high. The flows have been
'a'a and have flowed towards the west to the caldera wall and then turned
north. These flows have advanced episodically as pressure builds from ponding
and thickening of the flows. On April 13, a second 'a'a flow began to cover an
earlier, stagnant 'a'a flow and was almost entirely on top of the earlier flow
until April 15, when it spread out and destroyed another home. Cinder fell as
far as two kilometers south of the vents on April 15. By the 17th, the flow had
advanced to within 420 meters of the nearest house in the village of Portela,
but was stagnant again.

On April 18, the portable sesismometer installed by the U.S. Geological Survey
team began to record much stonger volcanic tremor (continuous ground shaking)
that indicated a change in eruptive style from fire fountains back to
Strombolian activity. Spatter was discharged every 3-8 seconds with loud gas
bursts. In addition, the eruptive rate increased and the upper 300 meters of
the channel was largely pahoehoe lava. The eruption rates, estimated from the
dimensions and flow rates in the channel, are between 4 and 8.5 million cubic
meters per day. For comparison, the current eruption at Kilauea has been
producing about one-half million cubic meters per day. In comparison, eruption
rates exceeded 10 and 20 million cubic meters per day during the high-fountain
phases of the Kilauea eruption (1983-1986) and the initial phases of the 1984
Mauna Loa eruption, respectively. Despite the increased eruption rate at Fogo
during this time period, the flow fronts advanced only a few meters as the
flows thickened.

During the night of April 18, a series of large, explosive bursts occurred
that may have followed collapse of parts of the cone into the vents and
subsequent explosive clearing of the debris. By morning, the activity had
returned to fire fountains, and the seismic record was much quieter. The lava
flows continued to thicken, and lava ponded along the central channel. As
pressure builds within the flow, a rapid advance of the flow into Portela seems
likely.

The structure of Fogo, with the caldera open to the sea on the east side, is
caused by processes similar to those that shape Hawaiian and other basaltic
volcanoes. The structure results from the sliding of the east flank of the
volcano in much the same way that the south flank of Kilauea slides towards the
ocean. In Hawaii, the most similar stucture can be seen on Kohala Volcano,
where a landslide structure has moved the northeastern side of the volcano down
and away from the summit. The indentation in the coastline between Waipio
Valley and Akoakoa Point mark the edges of the slide. Reunion Island in the
Indian Ocean has a structure most similar to Fogo, although Reunion consists of
an older western volcano and an active eastern volcano named Piton de la
Fournaise, which has a structure very similar to that of Fogo. It, too, has
been shaped by landslides that breached its caldera on the east side, and, like
Fogo, has had numerous historic-aged lava flows that reached the sea along its
eastern coast.

Our U.S. Geological Survey scientists are presently training several Cape
Verde geologists in the use and interpretation of the seismometer records, and
in the assessment of the eruptive activity and lava flows for future hazards
mitigation. These flows are presently ponding within the western part of the
caldera, but if the eruption were to continue at similar eruptive rates for
another month or so, the lava flows could thicken enough and spill out of the
caldera on the east side and advance to the coast.

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26 March 1998